In a transmitter having a plurality of transmission channels, carrier signal power of other transmission channels, especially of adjacent channels, leak to a transmission channel in question. In such a situation, a ratio between such power leaked from the adjacent channels and the carrier signal power of the channel in question is important for a purpose of evaluating transmission quality. Hereafter, the carrier power of the adjacent channels leaked to the channel in question may also be referred to as "leakage power", and the channel in question may also be referred to as "carrier channel" or "transmission channel."
Traditionally, when measuring leakage power from the adjacent channels of a transmitter, evaluation of signals in the steady state is performed in the frequency domain by, for example, a frequency spectrum analyzer.
FIG. 7 shows an example of waveforms with respect to a transmission (carrier) channel and a channel adjacent to the transmission channel. In a typical transmission channel, information data is transmitted for a constant period of time as a burst (shown in FIG. 7(a)), using a time-sharing transmission method. The burst-on period is typically referred to as a slot, while the repeated sequence of pulses between the bursts is typically referred to as a frame. For example, when an instantaneously changing signal, such as a TDMA (time division multiple access) burst signal is produced in the transmission channel, an interference wave is generally induced in the adjacent channels (i.e., the channels assigned to the neighboring frequency band).
FIG. 7(b) illustrates an example of a leakage-power waveform in the adjacent channel. Because each channel is assigned with a carrier signal whose frequency is different from the other, the power leaked from the adjacent channel has a frequency component different from that of the carrier-power of the channel in consideration. Leakage power is generated in the second, third and higher channels above and below the transmission channel. A maximum value in the adjacent channel constitutes the peak power.
FIG. 8 shows an example of conventional process for measuring leakage power. First, an average power level is measured using a power meter for each of the time periods of the burst-on period and the burst-off period. Subsequently, the average power during the burst-on period is computed by considering the duty ratio of the burst-on period with respect to the total period. The leakage power value determined by this method is an average value; therefore, the peak leakage power as shown in FIG. 7(b) cannot be determined.
In another conventional measurement method, a frequency domain test instrument, such as a spectrum analyzer is used. This method calculates the average power over the burst-on period after taking measurements in the frequency domain using a commercially available spectrum analyzer.
However, measurement results obtained by using a spectrum analyzer involve a process of sampling the input signal waveform. Generally, in order to ensure that the maximum power can be definitely determined, more than one sample per frame is required. For this reason, taking the sweep time into account, 90 millisecond is required to obtain one sample if the time length of one slot is 15 millisecond and one frame comprises 6 slots. If the spectrum analyzer takes 500 samples, it will take 45 seconds for one measurement of the peak leakage power. Therefore, such a measurement method takes too long and is not effective.
Recently, the necessity has arisen to measure an instantaneous event such as an interference wave at the rise and fall of a burst wave in a TDMA transmission. However, the above-described power meter or spectrum analyzer enables only the measurement of an average value of leakage power of one frame in the adjacent channels, and cannot measure an instantaneous event such as a TDMA burst signal.